1. Field of the Invention
The present invention relates to an image forming apparatus that employs the inkjet system or the electrophotographic system or the like as an output device such as an image recording apparatus that has the functions of a copier and a printer or a multifunction machine or workstation having a combination of the aforementioned functions. In particular, the present invention relates to a method for measuring the color of an image.
2. Description of the Related Art
In recent years, there is a demand for color image forming apparatuses such as color printers and color copiers to enhance the image quality of an output image. In particular, the stability of the image gradation or the image color significantly influences the quality of an image. However, in the case of color image forming apparatuses, the tints of obtained images vary in some cases due to environmental changes such as in the temperature or humidity, lot differences that are due to residual quantities or substitution of consumables or the like, the media that is used (the kind of recording material), and use of the apparatus over an extended time period. Therefore, to realize stable tints in output images, it is effective to detect the tints of output images using a color measurement device, and to feed back the detected result to the process conditions of the image forming apparatus. A color measurement device measures the tint (chromaticity) of a printed material or of the color of an object. Available color measurement devices include, for example, a tristimulus value direct-reading type color measurement device which irradiates white light at a color measurement object, and receives the reflected light with a light-receiving sensor through RGB color filters to thereby measure the intensity of each color component. The following spectral color measurement device is also available. That is, a spectral color measurement device is known which disperses the wavelengths of the reflected light using a diffraction grating or a prism, and thereafter detects the intensity of each wavelength with a line sensor. Next, the spectral color measurement device determines the spectral reflectivity of the color measurement object by performing a calculation that takes into account the wavelength distribution of the dispersed light that was detected, the wavelength distribution of the light of a light source, and the spectral sensitivity of the sensor.
This kind of spectral color measurement device has a configuration as illustrated, for example, in FIG. 10A. A detailed description of FIG. 10A is provided later. Using this kind of spectral color measurement device, a reference sample for which the chromaticity or spectral reflectivity is known is measured in advance, and after calibrating the apparatus itself so as to output the known chromaticity or spectral reflectivity, a color measurement object 104 is measured. In this case, as the reference sample, for example, a ceramic tile is used that has a white glaze applied on the surface layer thereof, as represented by a BCRA tile that is certified by the British Ceramic Research Association (BCRA). In addition, in the case of a simple color measurement device that outputs only the chromaticity, a white resin sheet that contains titanium oxide that has a spectral reflectivity as illustrated in FIG. 10C is used. FIG. 10C is described in detail later. In a color image forming apparatus equipped with a spectral color measurement device, detected results are fed back to a calibration table or the like for correcting an exposure amount, process conditions or a density-gradation characteristic of an image forming section. It is thereby possible to control the density or chromaticity of an image formed on a recording material.
In addition, a method for self-cleaning inside an image forming apparatus or processing for correction accompanying aged deterioration that are performed for the purpose of maintaining the color measurement accuracy of a color measurement device are proposed, for example, in Japanese Patent Application Laid-Open No. H11-216938 and Japanese Patent Application Laid-Open No. 2006-214968. For example, as in the case of a spectral color measurement device 100 illustrated in FIG. 10B, a configuration is known that includes a protective glass or a protective sheet for protecting the color measurement device from paper powder and dust that is generated inside the image forming apparatus. Because paper powder and dust adheres to the surface of a protective glass or protective sheet 111, for example, Japanese Patent Application Laid-Open No. H11-216938 proposes a method for cleaning the measurement surface of the color measurement device. The cleaning method described in Japanese Patent Application Laid-Open No. H11-216938 is effective for a configuration in which a gap between a reference sample that is disposed inside the image forming apparatus and the color measurement device is small (approximately 0.2 mm or less). Further, Japanese Patent Application Laid-Open No. 2006-214968 describes a method that focuses on the slope of a reference spectrum of a reference sample and the slope of a spectrum obtained by measuring the current reference sample, calculates a correction coefficient utilizing the inclination in the slope regions, and reflects the correction coefficient in a color measurement result. The method described in Japanese Patent Application Laid-Open No. 2006-214968 is effective with respect to deterioration of a light source and deterioration of a reference sample.
However, according to the method for cleaning the measurement surface of a color measurement device described in Japanese Patent Application Laid-Open No. H11-216938, in practice, the configuration that is adopted has a slight gap (approximately 0.2 mm to 2 mm) is provided in order to reduce the conveyance load when conveying a medium (recording material such as paper). Therefore, it is not possible to completely eliminate stains such as paper powder and dust on the color measurement surface of the color measurement device or the reference sample, and there is also the concern that mist or volatile colored gas or the like adhering to the color measurement surface or the reference sample cannot be removed. Further, according to the method described in Japanese Patent Application Laid-Open No. 2006-214968, variations in the amount of received light (variations in stray light) due to inner surface reflection that are caused by paper powder or colored adhering substances that adhere to the protective glass cannot be corrected when performing color measurement. That is, since the amount of received light increases when a large amount of return light is produced by adhering substances, when the same measurement object is measured, the difference in the return light is reflected as it is in the calculated result for the chromaticity that is obtained by the color measurement device. Therefore, particularly with respect to a measurement object that has a low lightness, there is the problem that the difference becomes more prominent and significantly influences the color measurement accuracy.